The invention relates to an electrolyte cell with an end anode and an end cathode and cell elements disposed between outer cell elements comprising these and electrically connected to them and connected in series with one another, wherein each cell element comprises one or two gas diffusion electrode(s), of which one forms simultaneously the ceiling of the subjacent electrolyte chamber and the floor of the superjacent electrolyte chamber as well as the end cathode and the anodes of the bipolar cell elements comprise a perforated, electrically well-conducting electrode structure, for example of nickel, wherein each electrolyte chamber is charged with electrolyte and reaction gas, such as oxygen, for example in the form of air, and a particular mixture of electrolyte and the resulting product as well as residual reaction gas is drawn off from each electrolyte chamber. Such an electrolyte cell has for example been suggested for the generation of ammonium polysulfide (APS) to which as the electrolyte is supplied an aqueous ammonium sulfide solution and from which is drawn off a solution comprising ammonium polysulfide. The electrolyte cell comprises an anode, a gas diffusion cathode and an electrolyte chamber disposed between the anode and the cathode. The cathode comprises an electrically conducting, gas-permeable carbon layer against which flows the gas comprising free oxygen and which is in contact with the electrolyte. Gas containing free oxygen is conducted into the electrolyte chamber and hyperoxide anions (OOHO--) are formed in it. From the electrolyte chamber are drawn off a solution containing ammonium polysulfide and a residual gas. Associated with the cathode is an areal, permeable metal element, for example a metal mesh or expanded metal, to which is connected a carbon layer. The carbon layer can be a carbon cloth coated with a mixture of graphite and PTF particles.
The use of a carbon cloth in gas diffusion electrodes is problematic in all application cases for the following reasons: into the gas diffusion electrode gas must diffuse, i.e. it must be porous. On the other hand, it is necessary to prevent gas from penetrating through the gas diffusion electrode since the desired reactions take place only on the surface of the electrolyte within the gas diffusion electrode. This means that fluid must also diffuse into the electrode. In the case of fuel cells this problem has been attempted to be solved thereby that the electrolyte was immobilized, i.e. a cloth or a felt was impregnated with the electrolyte. Thus, in each instance two porous cloths, namely for example the felt impregnated with the electrolyte and the gas diffusion electrode, are opposing one another in pressing contact with each moistening the other one, but not permitting penetration of the fluid.
In electrolyte processes in which substances dissolved in the electrolyte are to be converted and in which the solubility of the substances is limited, immobilizing the electrolyte is not possible. In the electrolytic generation of H.sub.2 O.sub.2 in an alkaline electrolyte, for example, the throughput of sodium hydroxide is determined by the solubility of Na.sub.2 O.sub.2 in the sodium hydroxide. If the carbon cloth on the side facing the fluid is made hydrophilic, and, on the side facing the gas, is made hydrophobic by impregnating the side facing the gas with PTFE, the electrolyte can penetrate up to the hydrophobic layer in the gas diffusion electrode. The gas can penetrate into the hydrophobic portion up to the hydrophilic portion filled with electrolyte. The reaction takes place at the boundary layer in the pores of the, overall, porous cloth. Due to differing capillary forces of the two different layers, however, the separating force is not unlimited. It is in general approximately 0.025 bar. However this also means that at a density of the electrolyte of 1 g/cm.sup.3 the height of one electrode is limited to 25 cm. This means further that the pressure difference in the reaction volumes can also not be greater than 0.025 bar. If it were greater, the gas would bubble through the upper portion of the perpendicular cloth. If it were less or if the electrolyte pressure were higher, the electrolyte would penetrate through the lower portion. In both cases the gas diffusion electrode would be inactive in these areas. In addition, capillary forces are a function of the adhesion of the fluid. Thus the electrolyte would demand a different level at each temperature. This, however, impairs the large-scale industrial application of this technology, in particular in extractive metallurgy. In the extraction of metals, the electrodes are, as a rule, taller than one meter. Flooding the gas diffusion electrode could for example only be avoided thereby that the metal extraction electrolysis is operated in vacuo such that electrolytes penetrating through the gas diffusion electrode are drawn off at the bottom from the electrolyte cell, for which additional expenditures are required.
Another possible solution of this problem comprises placing the electrodes horizontally. However, this cannot be realized in electrolyses for extracting metals since in this case open electrolyzer are used, from which metal in the form of solid cathode deposits must be removed at regular intervals.
Such an approach would, incidentally, fail for economic reasons. Horizontal cells are monopolar cells. Therefore, a multiplicity of cells would need to be electrically connected in series, such as, for example, in the case of the so-called mercury cell for generating chlorine and sodium hydroxide. In the present case this fails due to the current density which is far too low in gas diffusion electrodes. Due to their porosity the exchange of material is limited. As the limit of current density are generally considered 2 kA/m.sup.2. In mercury electrolyses for chlorine generation, in contrast, it is possible to work with current densities up to 15 kA/m.sup.2. At an equivalent production, the required area would thus increase by a multiple. For this reason, monopolar cells for large-scale industrial processes are in general equipped with vertical electrodes.
It is the task of the present invention to eliminate in an electrolyte cell of the above cited type the described disadvantages and to ensure reliable and continuous application at high efficiency.
This task is solved according to the invention for example thereby that the cell elements are combined in the form of a stack, that the end cathode and the cathodes of the bipolar cell elements comprise a perforated, electrically well-conducting support wall, for example of nickel, on which is disposed in each instance one gas diffusion electrode, and that the overflows provided at the cell elements are adjustable with respect to their height.
In this way it is possible to ensure that at no site of the electrolyte cell a hydrostatic pressure occurs which is higher than the penetration resistance of the gas diffusion electrode, implemented for example as a carbon cloth. The difference of the hydrostatic pressure from the penetration resistance of the gas diffusion electrolysis [electrode] can be such that, for example, the reaction gas flows sequentially through the cell elements at decreasing pressure. Such an interconnection contributes to the greater utilization of the gas. The stack of bipolar cells have a minimum space requirement; the number of superjacent cell elements is virtually unlimited. The required pipe lines are short. Due to the bipolar arrangement it is not necessary to use bus bars between the cell elements although the cell elements are electrically connected in series. Due to this interconnection the necessary electric energy is required at low current strength and high voltage which makes the transformers and rectifiers used cost-effective.
Due to the gas diffusion electrode, the invented electrolyte cell is primarily intended for the chemical conversion of oxygen at the surface of the aqueous electrolyte, to which is applied a voltage from the outside. The selectivity and the strength of the oxidation energy at the gas diffusion electrode can be set by means of the selection of suitable electrolytes but also by means of different catalysts, for example through platinum.
As such, the electrolyte cell can serve for example also as a fuel cell for generating energy if to both polarities gases reacting with each other are supplied from the outside. In contrast to conventional fuel cells, the value created is comprised in the reaction product generated. If oxygen generated at the anode is converted with hydrogen supplied from the outside, the electrolyte cell constructed in this way serves for decreasing the voltage and thus to save high-cost electric energy.
This is of interest especially if anodes with high oxygen overpressure can be replaced by gas diffusion electrodes. This applies, for example, to the electrolytic generation of metals with special emphasis on Zn and Cu. The invented electrolyte cell is preferably applied for the electrolytic generation of H.sub.2 O.sub.2 by oxidation of the H.sub.2 generated at the cathode with relatively complicated processes occurring in alkaline solution with O.sub.2 With a catalyst-free gas diffusion electrode the energy saving is approximately 0.6 V, the heat of formation of Na.sub.2 O.sub.2, which forms in alkaline solution, correspondingly approximately 450 kWh/t H.sub.2 O.sub.2.
A further technical application comprises the so-called Hydrina process in which Na.sub.2 SO.sub.4, which accumulates in large quantities as a neutralization product of H.sub.2 SO.sub.4 and NaOH, is again split into its starting products in the electrolysis. When using a gas diffusion electrode catalyzed with platinum as anode or as cathode and flow of H.sub.2, respectively O.sub.2, each over the electrode of opposite polarity, the following reaction equation results: EQU Na.sub.2 SO.sub.4 +3H.sub.2 O=H.sub.2 SO.sub.4 +2NaOH+H.sub.2 O-69.3kcal,
i.e. the energy requirement is limited to the heat of neutralization of acid and base.
With the electrolyte cell according to the invention the principle of the redox processes can basically be applied to any reduction/oxidation wherein the desired potentials can be set by means of different catalysts in the gas diffusion electrode, whereby the particular desired reaction proceeds as preferred.
Especially useful embodiments of the electrolyte cell according to the invention are in the dependent claims.
Whenever the oxidized product at the cathode can be reduced again, it is recommended to separate the anodes from the opposing cathodes by means of a diaphragm.
The diaphragm can therein be a cloth or a fiber mixture drawn onto the anode by suction, but it can also be an ion exchange membrane.
For specific application cases, for example in the production of H.sub.2 O.sub.2, the gas diffusion electrode can be free of catalyst.
For operation as a fuel cell, the anodes and the cathodes are advantageously covered with a gas diffusion electrode which is impregnated with the catalyst, and the cell elements are each divided by a gas-tight horizontal separating wall.
The particular cathode of the bipolar cell elements, in particular its electrode structure, can be connected via electrically conducting webs provided with penetration openings for gas, respectively gas and fluid, comprising for example nickel. When dividing the cell elements by a separating wall, a corresponding penetration opening is provided in the webs on each side of the separating wall.
To ensure economic construction of the electrolyte cell, the housings of the cell elements can be connected by means of edge flanges by interplacing electrically insulating seals such that they are gas- and fluid-tight.
Preferred functional operation results if the cell elements are connected in parallel with respect to the electrolytes. This operating manner is especially of advantage if low current strengths must be supplied in large quantities of electrolyte. This is for example the case when sterilizing water.
In the case of cell elements connected electrically and, with respect to the electrolytes, in series, preferably a particular mixture of electrolyte and the resulting product as well as reaction gas can be transferred from the electrolyte chamber of an upper cell element via connection lines to the electrolyte chamber of a lower cell element, and from the lowest cell element can be drawn off electrolyte and product and potentially residual reaction gas.
The connection lines associated with the overflows usefully terminate laterally in the particular lower cell element such that simple assembly is possible.
Further goals, characteristics, advantages and application feasibilities of the invention are evident in the following description of embodiment examples in conjunction with the drawing. All described and/or graphically depicted characteristics form by themselves or in any combination the subject matter of the invention even independently of their summary in the claims or their reference back.